Stable and Radioactive Isotopes in Precipitation Throughout Jordan

Summary

Monthly composite of the rain from 11 rainfall station from different part of Jordan were collected and analyzed for environmental stable isotopes (oxygen-18 and deuterium, D), and radioactive isotope (3H, tritium) for the period of 1987-1989.

The amount and distribution of rainfall in Jordan is of special importance in the appraisal of groundwater resources because of the rather scarce infiltration of precipitation is the main source of groundwater recharge.

Understanding the relationship between the variation of oxygen-18 and deuterium content of precipitation during its evolution through primary evaporation, condensation, re-evaporation, and infiltration is essential for studying groundwater origin. Isotopic models of precipitation are generally based on the Rayleigh fractionation mechanism: as rain or snow falls from the cloud, a depletion in oxygen-18 and D is observed in the residual condensed air mass and thus in subsequent precipitation.

On a global average, the general relation between oxygen-18 and D for natural waters is found to be linear and can be expressed by the following equation (Craig, 1961).

D = 8 * oxygen-18 + 10 o/oo

The deviation from this global meteoric water line (GMWL), called local meteoric water line (LMWL) and can be expressed through the deuterium excess parameter (d). The d-parameter is defined as (Dansgaard, 1964):

d = D - 8*oxygen-18

The location of the data on the LMWL indicates the origin of the air moisture. Precipitation Throughout the Eastern Mediterranean area, shows a different correlation between oxygen-18 and D, namely, d ~ 22 o/oo (Gat and Carmi, 1970).

The table below shows the mean weighted value of all the environmental isotope samples from Jordan..

Name	Oxygen-18 o/oo	Deuterium o/oo	Tritium T.U.	No. of samples
Azraq	-4.64	-20.02	11.05	20
Irbed	-6.56	-29.51	8.22	25
Ras Munif	-7.25	-33.19	8.90	15
Baqa	-5.85	-21.24	9.13	12
Amman	-6.47	-27.72	10.51	40
Q.A.Airport	-5.86	-23.57	9.79	13
Walla	-5.35	-22.07	13.25	13
Rabba	-5.13	-19.90	19.61	28
Shobak	-6.56	-28.92	8.83	21
Deir Alla	-4.04	-14.05	10.17	11
Aqaba	-3.52	-11.52	8.50	4

The relationship between the variation of oxygen-18 and D for precipitation are shown in Fig.1. The least square line fitted to all the value of oxygen-18 and D is:

D = 6.53 * oxygen-18 + 13.65 (r=0.93)

The WMV of precipitation at all the station fall close to the EMWL. Notice however that the samples of Azraq, Deir Alla and Aqaba show deviation from the EMWL. The oxygen-18 and D of the precipitation at these stations were subject to the amount and temperature effects. There is also a tendency that with high altitude the oxygen-18 value of precipitation is more depleted than low altitude, this spatial variation in the stable isotope composition is also of practical significance for hydrogeological applications. Figure 1 shows the evolution of the isotopic composition of rain with increasing altitude from the Jordan Rift valley (Deir Alla Station, -224 m below sea level) to the various topographic to the west (Ras Munif, Amman, Rabba, 1150 m, 900 m, 950 m above sea level respectively) along the movement of air mass. A continuous depletion in heavy isotope of air masses during their ascent and passage over Eastern Escarpment is observed. In Deir Alla the oxygen-18 is -4.04 o/oo while in Ras Munif the oxygen-18 is -7.26 o/oo.

From the above we can conclude that the Mediterranean Sea is the dominant vapor source for the regional precipitation. These features of the isotope distribution give rise to a model which is based on the air mass movement into the Mediterranean area during winter, when cold and dry continental air masses come in contact with warm sea, resulting in rapid evaporation and large scale convergence (cyclo-genesis).

Tritium is a radioactive isotope of hydrogen (3H), has a half life of 12.43 years. Tritium is produced in the upper atmosphere by interaction of cosmic ray produced neutrons with nitrogen atoms nitrogen-14 + n ---> 3H + carbon-12. One tritium unit (T.U.) correspond to 0.118 Bq/Kg.

Since 1952, the tritium content of the atmosphere has increased greatly owing to thermonuclear explosion in the northern hemisphere. Maximum levels of this tracer recorded up to 10,000 T.U in April, 1963 at Whitehorse, (Canada), following extensive testing in 1961-1962.

Tritium can be used to elucidate the residence time of recently recharged groundwater. The available data of tritium of precipitation in Jordan from the period of 1965 (IAEA, 1969) and 1987-1989 (Bajjali, 1990), indicates a general decreasing trend of tritium levels in precipitation water with time, which reflects the global trend following the ban on the atmospheric testing of hydrogen bombs..

Jordan lacks long term tritium measurements. Therefore, a correlation between the closest station to Jordan (Bet Dagan, Israel) with relatively long term measurements was taken as an indicator of tritium values for Jordan. This station has a well established correlation with Ottawa station (Canada). Ottawa has the longest term tritium record in the world, link to (ftp://ftp.iaea.or.at/pub/gnip, INTERNET). This correlation shows that the highest level of tritium observed in the area was around 600 T.U in 1963.

The average tritium concentration in the atmosphere from the 11 rainfall stations for the winter season 1987/1989 is approximately 11 T.U. We can observe from the table above that the tritium concentration is more or less identical for all the stations. The exception is Rabba station, the WMV of tritium is higher than the other ten rainfall stations.

The tritium concentration in the precipitation in Rabba still two fold higher than the average of tritium throughout the country. If we examine carefully the behavior of the environmental isotope we find that the stable isotope values of the precipitation in Rabba station for the period of 1987/1989 is responding like the other stations in Jordan to the amount, temperature, and altitude effects. The environmental radioactive tritium values of the precipitation in Rabba for the same period does not correspond to these parameters. The main question remains to answer is from where this access source of tritium. Furthermore, we can not link the high tritium value in the atmosphere above Rabba with the uranium content found in the Upper Cretaceous limestone in the area. Because the end product of the disintegration of Uranium doesn't lead to tritium: